1. Field of the Invention
The present invention relates to a method and apparatus for detecting a watermark. The invention has particular application to detecting a watermark in a digital picture or sound signal.
2. Description of the Related Art
Watermarking is a well known technique which is used to protect against the fraudulent copying or counterfeiting of documents or currency. The physical medium carrying the document or currency is marked with a distinctive and recognizable mark which is difficult to remove. In more recent time a need has arisen to protect digital signals representing picture information or audio information by means of a distinctive watermark. A watermark may be used to establish the true origin or ownership of the picture or audio information that is represented by the signals. It can be of particular benefit in the fields of digitally broadcast television signals and digitally recorded picture or sound signals.
The watermarking of documents or currency requires the watermark to be readily visible to assure the person examining the document or currency that the document or currency is genuine and has not been counterfeited. A physical technique of this sort is of course inapplicable to the protection of digital signals. Digital signals can be manipulated using digital techniques thus removing an existing watermark and possibly introducing a fresh watermark. The watermark for protecting a digital signal therefore should be obscured from an observer or listener of the information represented by the digital signal and should be difficult if not impossible to remove. The watermark should still be detectable by the originator of the watermark to determine if a signal is genuine.
In addition it is advantageous that the watermarking for digital signals be sufficiently robust so as to be capable of withstanding the compression and decompression common in digital broadcasting techniques while still reliably indicating that the signal has been watermarked.
Existing watermarking schemes use a technique in which a watermark is generated from a random number sequence and added to the original picture or sound information to form a watermarked signal in which the watermark is obscured. When it is desired to detect the presence of the watermark, the watermarked signal is correlated with the watermark to generate a correlation signal which reveals the watermark.
A problem with the existing technique is that the watermark which is revealed through the correlation may be difficult to perceive. This may be due in part to the introduction of noise into the watermarked signal and may also be due to the difficulty in distinguishing the watermark from the information represented by the signal.
A need therefore exists to improve the correlation for detecting the watermark.
According to the present invention there is now provided a method of detecting the presence of a watermark in input digital data, the watermark including coefficients embedded in the data, the method comprising the steps of, transforming the data and applying the transformed data as a first input and the watermark coefficients as a second input to a cross correlator so as to generate an output detection signal, characterised in that the method further includes scaling the cross correlation by a predetermined characteristic of the data.
The invention has the advantage that the output detection signal is significantly less noisy than in a conventional detector.
Further according to the present invention, there is provided apparatus to detect the presence of a watermark in input digital data, the watermark including coefficients embedded in the input data, the apparatus comprising means to receive and transform the input data, a cross correlator having a first input to receive the transformed data and a second input to receive the watermark coefficients, the cross correlator being effective to generate an output detection signal, characterised in that scaling means are provided to scale the cross correlation by a predetermined characteristic of the data.